Harmonic drive and internal geared wheel for a drive of this type

ABSTRACT

The invention relates to a harmonic drive ( 12 ) which can be provided with overload protection that is easy to influence constructionally and a free-wheeling characteristic. To this end, the spokes ( 21 ) of the internal geared wheel ( 15 ) are specifically deformable depending on the torque or the direction of rotation. The spoke shafts ( 22 ) are divided crosswise to the drive axis ( 24 ) or in a radial direction in relation thereto. This division ( 45 ) is bridged over flexibly in the peripheral direction of the internal geared wheel so that the effective radial length of the spoke may be reduced in the presence of the appropriate bending stress by buckling the shaft, in order to then eliminate the local toothing contact between the internal geared wheel edges ( 13 ) and the outer support ring ( 11 ) in the extension of the spoke. The bridging of the spoke division can be configured differently for the two directions of rotation in order to obtain different response characteristics, up to direction-dependent free-wheeling characteristics e.g. in the event of a considerably unsymmetrical spoke shaft course in relation to the radials.

[0001] The invention relates to a harmonic drive and internal gearedwheels for such drives according to the respective preamble of the mainclaims, as known in this respect from DE 1 96 50 716 C1 relating to aneccentric drive with overload protection.

[0002] The function of such an eccentric or harmonic drive—also known asa circular spline drive—as a very greatly reducing, self-locking systemwith an output shaft coaxial with the drive shaft is based on the factthat a rotating wave generator radially deforms an internal geared wheelrim, also referred to as a flexspline, orbitally and in this wayorbitally presses its outer lateral surface outward locally against thehollow-cylindrical inner lateral surface of slightly greatercircumference of a stationary, dimensionally stable support ring.Consequently, the internal geared wheel itself or its wheel rim mountedrotatably thereon rolls in the support ring non-positively via frictionsurfaces or positively via toothings, the wheel or its rim rotating moreslowly than the motor-driven driving core of the wave generatoraccording to the difference in circumference. This rotary motion, whichis greatly slowed in relation to the drive, is preferably transmittedvia the outer toothing of the wheel rim to the inner toothing of afurther hollow-cylindrical outer ring of the output ring which isconcentric with the support ring but not stationary. The drive of thewave generator usually takes place via a coaxially flanged-onhigh-speed, and therefore inexpensively procurable, extra-low-voltaged.c. motor, the rotation of which is thus reduced to a very much slowerrotary motion of correspondingly greater torque.

[0003] In the embodiments of such harmonic drives known from GermanUtility Model 2 96 14 738 and described in greater detail in the article“Genial einfach [Ingeniously simple]” by H. Hirn (KEM Antriebstechnik,issue November 1996), a non-round (in axial cross section roughlytriangular or preferably oval) driving core, as the wave generator, isrotated concentrically in the hub of the radially deformable internalgeared wheel. Dimensionally stable spokes, acting as radially orientedpush rods, between the hub deformed radially by the driving core and thelikewise radially deformable, externally toothed rim of this internalgeared wheel cause the outer toothing, according to its orbital radialdeformation, to enter into engagement with the inner toothing of thesupport ring at any one time over only a limited curved segment rollingtherein.

[0004] The dimensional stability of the spokes must be guaranteed aboveall in the radial direction in order for it to be possible to transmitthe orbital radial deformation of the hub (referred to as an internalgeared wheel in the prior publication constituting the generic type)generated by the wave generator to the rim (referred to as the externalgeared wheel there). However, during the rolling operation of the rim inthe support ring, which takes place under load, the spokes of thissegment, which are linear there and described as flexible in theperipheral direction, are also subjected to bending stress at both theirends. If they then bend in the peripheral direction out of the radialdirection, the diameter of the rim decreases until it comes out ofengagement with the support ring. The spokes thus act as overloadprotection on the harmonic drive.

[0005] The invention is based on the technical problem of, whileretaining its advantages in terms of apparatus and application, todevelop the previously known harmonic drive or its internal geared wheelto the effect that it can be designed considerably more flexibly inparticular with regard to the kinetic conditions when the overloadprotection responds and will thus have a greater variety of uses.

[0006] According to the invention, this object is achieved by theharmonic drive according to the generic type and its internal gearedwheel being designed with the features of the respective main claim.Accordingly, the internal geared wheel is still equipped with spokes,which are no longer dimensionally stable in the peripheral direction atcritical stress, a torque-dependent—and optionally arotation-direction-dependent—response of this safeguarding functionagainst overloading then being specifically constructionallyinfluencable with regard to many parameters owing to the special shapingor material combination, according to the invention, of the spokes. Asmentioned, such restorable deformation occurs depending on the flexuralrigidity of the spoke when that segment of the wheel rim assigned to itsradial orientation is directly in engagement with the support ring and aconstructionally specified critical bending stress of this spoke is onthat account exceeded during the rolling operation.

[0007] In this connection, the deformability of the spokes can thereforebe designed so as to be independent of the direction of rotation of thedriving core in the wheel hub, so that, as it were, the function of anoverload protection or slipping clutch acting in both drive directionsis brought about. On the other hand, this function can also be definedso as to be dependent on the direction of rotation by the spoke having,owing to its geometry or material combination, a different flexuralrigidity in one direction of rotation than in the opposite one—up to thefree-wheeling characteristic in one direction with power transmission inonly the specified other direction, the working direction. This isbecause the deformation of the spoke shaft, which is subject to overloador which is now no longer flexurally rigid in the counter-rotationdirection, shortens the radially effective spoke length of the internalgeared wheel and thus the local radial deformation of the wheel rim,that is to say its toothing engagement with the support wheel, whichthen ratchets through and thus comes out of engagement, as a result ofwhich the power and motion transmission via the harmonic drive isinterrupted.

[0008] Consequently, by means of the measures according to theinvention, that is to say as a result of influencing the toothingengagement, the toothing parameter on the wheel rim (flexspline) ismodified in such a way that, when the specified maximum torque isexceeded, ratcheting-over of the toothing occurs. This leads to thedrive coming to a standstill in the event of overload, in whichconnection this torque-limiting by means of buckling spokes up tofree-wheeling functioning owing to spokes being inclined in relation tothe radials or owing to spokes folding down can also bedirection-dependent, without additional functional assemblies having tobe introduced for this before or after the harmonic drive.

[0009] With regard to other details and additional developments of theinvention, reference is made to the following description of preferredillustrative embodiments of the solution according to the invention andto the other claims and also, to supplement the disclosure, to our twocurrent German patent applications with the same title. In the drawing,which is enlarged not entirely to scale and simplified to the essentialin terms of function, with very rough toothing geometry:

[0010]FIG. 1 shows the upper half of a wheel rim, which rolls in thesupport ring fixed relative to the housing, on a harmonic drive internalgeared wheel with solid parallelepipedal spokes which are flexurallynon-rigid, if appropriate in a rotation-direction-dependent manner,owing to transverse division of their shaft in an axially parallelmanner relative to the drive axis,

[0011]FIG. 2 shows part spokes from FIG. 1 coupled (a) elastically onboth sides (b) singly elastically and (c) pivotably on one side,

[0012]FIG. 3 shows in modification of FIG. 2(a) the lower half of awheel rim, which rolls in the support ring fixed relative to thehousing, on a harmonic drive internal geared wheel with spokes which areflexurally non-rigid, if appropriate in a rotation-direction-dependentmanner, owing to slots which are radial relative to the drive axis, and

[0013]FIGS. 4 and 5 show in modification of FIG. 3 the upper and,respectively, in modification of FIG. 4 the lower half of a wheel rim,which rolls on a support ring fixed relative to the housing, on aharmonic drive internal geared wheel with a spoke which is flexurallynon-rigid in a rotation-direction-dependent manner owing to a shapewhich is curved in relation to the radial direction toward the driveaxis.

[0014] The drawing illustrates, in each case in cutaway axial section,views toward the end sides 13 of various internal geared wheels 15, eachequipped with a plurality of identical spokes 21, inside their supportring 11, which is in each case fixed relative to the housing anddimensionally stably hollow-cylindrical, of harmonic drives 12.

[0015] It is true for each of the harmonic drives 12 illustrated thatits non-round (preferably oval in axial cross section as illustrated)dimensionally stable driving core 14 extends coaxially through theradially deformable internal geared wheel 15 with its wheel rim 13. Inthe driving core 14 in turn, which is rotatable concentrically in theinternal geared wheel 15, a drive shaft 16, which may directly be thelengthened motor shaft of a drive means (not visible in the sectionalillustration, located concentrically above the drawing plane in the formof, for instance, an electric or fluid motor), engages, again coaxiallywith the drive axis 24 but in this case in a rotationally fixed manner.

[0016] The wheel rim 13, which is deformed orbitally in a non-roundmanner in cross section by the driving core 14 via the radial push-rodaction of the spokes 21, is rotationally connected to the dimensionallystably hollow-cylindrical inner lateral surface 18 of the stationarysupport ring 11 non-positively or, as illustrated, positively via aradial toothing 19/20 only in extension of the major radii of thedriving core 14 along that curved segment of its outer lateral surface17 which is displaced radially outward sufficiently far there. When thedriving core 14 rotates, the orbitally radially deformed outer toothing19 rolls in the support ring 11 and, on account of its smallercircumference, performs less than a full revolution with each revolutionof the driving core 14. In this connection, the rim 13 can be rotatablerelative to the internal geared wheel 15, and the internal geared wheel15 can also stand still completely.

[0017] At least this externally toothed rim 13 extends with its toothing19 axially below the drawing plane and out of the support ring 11 into afurther likewise dimensionally stably hollow-cylindrical outer ringwhich is axially adjacent to the support ring 11 visible in the drawingbut in this case is not arranged in a stationary manner but is mounted,as the coaxial output ring of this very greatly reducing drive 12,corotatably by the toothing 19 of the rim 13. In order that the outerrings (support ring 11 and output ring), which, like the internal gearedwheel 15, are likewise usually produced by injection-molding plastic, donot widen radially under load and then allow their respective toothingto ratchet through in relation to the rim 13, they is are appropriatelydimensionally stabilized by reinforcing rings made of, for instance,metal, which are applied subsequently or, better still, molded instraightaway (not visible from the sectional plane of the drawing).

[0018] The axially long and radially wide radially deformable internalgeared wheel 15 is assembled from a number of identical movablyinterconnected push-rod-like spokes 21 lying next to one another in aray-like manner. Each of these per se dimensionally stable spokes 21consists first and foremost of a parallelepipedal shaft 22 which extendsroughly over the axial length of the driving core 14 and thus of theinternal geared wheel 15 and of its rim 13. Either the shaft is,according to FIG. 1, supported by its foot 23 directly along thegeneratrix and, in relation to the drive axis 24, radially on thecross-sectionally non-round outer lateral surface of the driving core14, or the foot 23 emerges radially from a radially deformable wheel hub25, as in FIG. 3. The contact region of each foot 23 and, as the casemay be, the inner lateral surface of the hub 25 are expediently equippedwith a dimensionally stable, wear-resistant and temperature-resistantcoating 42 or insert which slides readily in relation to the material ofthe driving core lateral surface.

[0019] Opposite the foot 23, each of the per se dimensionally stableT-shaped spoke shafts 22 ends in a—preferably somewhat radiallyelastically compressible—head 26 which bears the rim 13 in arotationally rigid manner or over which the rim 13 runs with goodsliding properties. The head 26 has essentially the cross-sectionalshape of a cylinder sector or hollow-cylinder sector extending axiallyparallel over the length of the driving core 14 and thus the axiallyparallel length of the shaft 22. In contrast to this illustrativeembodiment, however, a separate rim 13 which can be locally radiallywidened orbitally does not have to be borne by the internal geared wheel15, but the spaced peripheral sequence of the heads 26 can also servedirectly as the outer lateral surface 17 of an internal geared wheel 15with a wheel rim (13) formed in one piece therewith, which surface isdiscontinuous in the peripheral direction, equipped with outer toothing19 and is pressed radially into the inner lateral surface 18 of thesupport ring 11 orbitally.

[0020] The lateral support of the spokes 21 in their peripheral sequencein relation to one another, and thus at the same time their circularpositioning and movable connection to form the assembled internal gearedwheel 15, can be brought about by means of short arms 28 formedlaterally opposite one another and preferably in one piece on therespective shaft 22 (according to FIG. 1, for instance), but also,instead of or in addition to this, by means of the positioning of thespokes 21 along the circumference of the wheel hub 25 (cf. FIG. 3).

[0021] During operation of the drive 12, the spokes 21, which primarilytransmit the orbital radial deformation force from the driving core 14to the rim 13 in a push-rod-like manner, are also subjected to bendingstress depending on the direction of rotation of the driving core 14.According to the invention, the torque which can be transmitted via thedrive 12 is in this case limited by the flexural rigidity of the spokes21 being specifically weakened. According to FIG. 1 and FIG. 2, it is tothis end envisaged to divide the parallelepipedal shafts 22 of thespokes 21 parallel to the drive axis 24 roughly centrally, that is tosay roughly halfway along their radial longitudinal extent, and in eachcase to interconnect the two part shafts across this separating gap 45by means of a flexible coupling 46. For FIGS. 2(a) and (b), elasticallyextensible couplings 46 are provided for these bridges, two of which arearranged, at the two ends of the dividing gap 45, in the former, and oneof which is arranged in the extent of the dividing gap 45 in the latter.In this way, pivoting (as indicated by the arrows in the drawing) of thetwo parts of the shaft 22 in relation to one another, with mutualsupport of the end faces opposite one another at the dividing gap 45, ismade possible when the torque stress between the driving core 14 and thesupport ring 11 exceeds a constructionally specified response value.This response value can be influenced via the position, the directionand the geometry of the dividing gap 45 in the shaft 22 and also via theextension behavior of the couplings 46 relative to one another (a) orvia the position of the coupling in the interior of the dividing gap 45(b) . In this way, it is also possible to specify that the load limit inone direction of rotation is different than in the opposite direction ofrotation, so as, for instance during operation of an installation, tolimit the working load to a function-critical value but, in the event ofa fault occurring, to ensure movement back under full power. For purefree-wheeling functioning, all that is necessary, according to FIG.2(c), is to design a coupling without elasticity at that end of thedividing gap 45 lying in the direction of rotation, in other words asimple bending hinge which, when a backward motion takes place (in theopposite direction to the arrow in the drawing), allows the end faces onboth sides of the dividing gap 45 to fold apart and in this way, owingto shortening of the effective radial length of the spoke 21, takes thetoothing 19/20 out of engagement. The elastic or only flexurallynon-rigid couplings 46 for bridging the dividing gaps 45 can be appliedin the form of separate construction elements but they can also beintegrated simply in the form of film strips between the part shafts 22in the course of manufacture by injection molding.

[0022] In the variant according to FIG. 3, the shaft 22 of the spokes 21is, in contrast to FIGS. 1/2(a, b, c), no longer divided transversely totheir longitudinal extent but, in modification of FIG. 2(a), is dividedlongitudinally over part of their radial longitudinal extent to form twolateral branches 47 so as to bring about a determinately flexurallyrigid zone roughly in the central region of the shaft 22. This zone canagain, in this case by means of, for instance, different thickness ofthe two parallel branches 47 of the longitudinally divided shaft 22, bedimensioned differently for the two directions of rotation.

[0023] In an extreme case, one of the parallel branches 47 is omittedentirely, so that geometries according to, for instance, FIG. 4 andfinally the shape of the spokes 21 according to FIG. 5, curved in aleading manner in the working direction of rotation, are obtained forthe shafts 22. These are internal geared wheels 15 with a free-wheelingcharacteristic in the opposite direction of rotation, which does not(like the working direction indicated by the respective arrow) lead to aradial spreading of the outer lateral surface 17 for the localengagement of the rim 13 in the support ring 11. In this connection, theresponse behavior of the free-wheeling characteristic when the torque isreversed can also be constructionally influenced, in addition to via thegeometry of the leading shape of these spokes 21, via the radialposition in the drive cross section and via the rigidity of theperipheral connecting arms 28 between the spokes 21.

[0024] According to the. invention, a harmonic drive 12 can thus beequipped with a constructionally easily influencable overload protectionand free-wheeling characteristic by means of spokes 21 of its internalgeared wheel 15 which are specifically deformable in a torque-dependentor rotation-direction-dependent manner, that is to say which areflexurally non-rigid. To this end, the spoke shafts 22 are dividedtransversely to or in the radial direction in relation to the drive axis24. This division is bridged flexibly in the peripheral direction of theinternal geared wheel 15 so as, when corresponding bending stressoccurs, to reduce the effective length of the spoke 21 in the radialdirection in relation to the axis 24 by buckling the shaft 22 and thusto discontinue the local toothing engagement 19-20, in extension of thespoke 21, between the internal geared wheel rim 13 and the outer supportring 11. The couplings 46 or branches 47 for bridging the spoke divisioncan be designed differently for the two directions of rotation in orderto achieve different response behavior up to the direction-dependentfree-wheeling characteristic in the case of spokes 21 which areflexurally rigid in only one stress direction, for example in the caseof a shape of the spoke shaft 22 which is greatly unsymmetrical inrelation to the radials.

1. A harmonic drive (12) with, rotating about its axis (24), a drivingcore (14) of non-round cross section for coaxial rolling of the wheelrim (13) of a radially deformable spoked-wheel-shaped internal gearedwheel (15), which surrounds the core, in a dimensionally stable supportring (11) of somewhat greater diameter, the internal geared wheel (15)being equipped, for torque limiting, with spokes (21) which areflexurally non-rigid in a torque-dependent manner, characterized in thatthe spokes (21) are divided transversely to the longitudinal extent oftheir shafts (22) by a flexibly bridged separating gap (45).
 2. Aninternal geared wheel (15), for a harmonic drive (12) as claimed inclaim 1, characterized in that it is equipped with spokes (21) dividedtransversely to the longitudinal extent of their shafts (22) by aflexibly bridged separating gap (45).
 3. The internal geared wheel asclaimed in the preceding claim, characterized in that the bridging hasat least one elastically extensible coupling (46).
 4. The internalgeared wheel as claimed in the preceding claim, characterized in thatthe bridging has at least one elastically extensible coupling (46) inthe extent of the separating gap (45).
 5. The internal geared wheel asclaimed in the preceding claim, characterized in that the bridging is atleast one flexible coupling (46) at at least one end of the separatinggap (45).
 6. The internal geared wheel as claimed in at least one of thethree preceding claims, characterized in that the coupling (46) respondsin a rotation-direction-dependent manner.
 7. The internal geared wheelas claimed in the preceding claim, characterized in that the bridginghas an elastically extensible coupling (46) in each case in and counterto the direction of rotation of the wheel rim (13), for instance at thetwo ends of the separating gap (45), which are of different design. 8.The internal geared wheel as claimed in one of the preceding claims,characterized in that the bridging is a plastic film.
 9. An internalgeared wheel (15), for a harmonic drive (12) as claimed in claim 1,characterized in that the spokes are divided into at least two parallelbranches (47) in the longitudinal direction of their shafts (22).
 10. Aninternal geared wheel (15), for a harmonic drive (12) as claimed inclaim 1, characterized in that the spokes (21) extend in a curved mannerout of the radials to its axis (24) in and/or counter to the directionof rotation of its wheel rim (13).